Method of removing sulphur dioxide from a gaseous stream

ABSTRACT

Capture-fixation of SO 2  from flue and non-ferrous smelter gases is achieved with the use of MgO aqueous slurry as an absorbing medium. During the absorption, most of the MgO is converted to MgSO 4  and the rest is solubilized as MgSO 3  and Mg(HSO 3 ) 2 . The clear solution obtained at the end of the absorption is brought to about 40° C. and is treated with sulphuric acid in an amount representing at least 5% in excess of the stoichiometric requirement whereby sulphite and bisulphite of magnesium are converted to magnesium sulphate. The magnesium sulphate obtained above can be used in a two stage addition of potassium chloride to obtain magnesium chloride, and calcined dolomite added to the remainder whereby magnesium hydroxide is precipitated and may be filtered off from the resulting CaCl 2  solution. The process provides a means of removing SO 2  from flue and smelter gases and at the same time produces industrially important chemical products (SO 2 , H 2  SO 4 , K 2  SO 4 , MgO, CaCl 2 ).

This invention relates to a method of removing sulphur dioxide from a gaseous stream.

It is well known to remove sulphur dioxide from waste gas using an aqueous slurry of magnesium oxide having a pH greater than 6 to produce insoluble MgSO₃, see for example Mathews, J. C. et al. "SO₂ control process for non-ferrous smelters"; Research Triangle Institute, distributed by NTIS as Report PB-251-409; 1976, pages 95 and 98.

While this known process is useful, it would be advantageous to provide a method of removing sulphur dioxide from waste gases wherein industrially important chemical products result from the process.

According to the present invention there is provided a method of removing sulphur dioxide (SO₂) from a gaseous stream, comprising:

(a) contacting the gaseous stream with an aqueous slurry of magnesium oxide (MgO) containing about 3 to 10%/w of magnesium oxide, so that SO₂ of the gaseous stream is absorbed by the aqueous slurry by converting MgO to an aqueous solution at a pH in the range of about 4.0 to 4.5 containing MgSO₄ and different smaller amounts of Mg(HSO₃)₂, H₂ SO₃ and water soluble MgS03, and

(b) thoroughly mixing sulphuric acid (H₂ SO₄) with the aqueous solution, at a H₂ SO₃ decomposition temperature of at least 40° C. to produce SO₂ and H₂ O therefrom, the sulphuric acid being present, in an amount representing at least 5% in excess of the total stoichiometric requirement of the aqueous solution to decompose Mg(HSO₃)₂ and MgSO₃ according to the equations:

    MgSO.sub.3 +H.sub.2 SO.sub.4 →MgSO.sub.4 +SO.sub.2 +H.sub.2 O, and

    Mg(HSO.sub.3).sub.2 +H.sub.2 SO.sub.4 →MgSO.sub.4 +2SO.sub.2 H.sub.2 O.

In some embodiments of the present invention the sulphur dioxide (SO₂) produced in step b) is converted to sulphuric acid by catalytic reaction with H₂ O and O₂ according to the equation:

    SO.sub.2 +1/2O.sub.2 +H.sub.2 O→H.sub.2 SO.sub.4.

In other embodiments of the present invention the magnesium sulphate produced in step (b) is converted to magnesium chloride by thoroughly mixing therewith potassium chloride and H₂ O according to the following two stage process:

(i) adding potassium chloride to produce the following reaction:

    2MgSO.sub.4 +2KCl+6H.sub.2 O→K.sub.2 SO.sub.4.6H.sub.2 O+MgCl.sub.2,

whereby the potassium sulphate (K₂ SO₄.MgSO₄.6H₂ O), in the form of a double salt, is separated from the aqueous solution of magnesium chloride, and then adding additional potassium chloride to the separated double salt suspended in water to produce the following reaction

    2KCl+K.sub.2 SO.sub.4.MgSO.sub.4. 6H.sub.2 O→2K.sub.2 SO.sub.4 +MgCl.sub.2 +6H.sub.2 O,

then separating the K₂ SO₄, which has crystallized from the solution, from the remainder.

Calcined dolomite (CaO.MgO) may be added to the remainder to produce the following reaction:

    MgCl.sub.2 +CaO.MgO+2H.sub.2 O→2Mg(OH).sub.2 +CaCl.sub.2 (aqueous solution),

and the magnesium hydroxide (Mg(OH)₂) separated from the calcium chloride (CaCl₂) aqueous solution. A portion of the magnesium hydroxide thus separated may be formed into an aqueous slurry and is recycled for use as the aqueous slurry of magnesium hydroxide in step a), and the remaining magnesium hydroxide is calcined to produce MgO therefrom.

Furthermore, a portion of the H₂ O may be removed from the calcium chloride (CaCl₂) aqueous solution to crystallize calcium chloride in the remaining water, and then the crystallized calcium chloride may be separated from the remaining water and dried.

The step (b) described above may be carried out under a reduced pressure.

In the accompanying drawings which illustrate, by way of example, embodiments of the present invention,

FIGS. 1 and 2 are diagrammatic views of experimental apparatus for removing SO₂ from a gaseous stream,

FIG. 3 is a mechanical flow diagram for removing SO₂ from a gaseous stream on an industrial scale, and

FIG. 4 is a flow diagram for an entire industrial process for the removal of SO₂ from a gaseous stream and converting SO₂ and MgSO₄ to different by-products (H₂ SO₄, MgO, CaCl₂).

Referring now to FIG. 1, there is shown a cylinder 1 containing pressurizedSO₂ connected to a sparger 2 by a tube 4 containing a control valve 6,a flow meter 8 and a check valve 10. A tube 12 containing a control valve 14, flow meter 16 and check valve 18, is connected to the tube 4 for feeding a pressurized source of air (not shown) thereto at a position beyond the check valve 10.

The tube 4 extends through, and seals the sparger 2 in a graduated beaker 22 by a rubber bung 20. A thermometer 24 extends into the beaker through the bung 20. A gas outlet tube 26 extends through the bung 20 and through another bung 28 into a flask 30. A gas outlet 32 from the flask 30 also extends through the bung 28.

The beaker 22 contains a magnetic stirrer 34 and is supported on an electrical heater 36 which also actuates the magnetic stirrer 34.

In operation, with the apparatus arranged as shown, a magnesium oxide slurry 38 is placed in the beaker 22 and a magnesium oxide slurry, or sodium hydroxide slurry, designated 40 is placed in the flask 30.

The electrical heater 36 is actuated to heat the slurry 38 and actuate the stirrer 34 and SO₂ and air are fed to the beaker 22 along the pipelines 4 and 12 to bubble through the slurry 38 from the sparger 2.

As the mixture of SO₂ and air bubble through the slurry 38, SO₂ is absorbed in the slurry 38 by converting MgO in the slurry to MgSO₄, Mg(HSO₃)₂, H₂ SO₃ and water soluble MgSO₃.

Any trace of SO₂ not absorbed in the slurry 30 passes along the pipeline 26 and is absorbed as it is bubbled through the slurry 40 in the flask 30.

In experiments to verify the present invention, different MgO slurry concentrations, ranging from about 3-10 wt %, were used in the beaker 22 and each slurry 38 concentration achieved the desired goal, i.e. to capture SO₂ from the air-SO₂ gas "mixture", at a temperature ranging up to about 50° C. depending on the SO₂ content in thegas mixture. The preferred slurry 38 concentration for the MgO scrubbing-KCl conversion process, however, was found to be about 8.27 wt %MgO at a temperature in the range 30° to 40° C. since it produced a solution that contained about 240-250 gr/liter MgSO₄ whichcould be used directly for the conversion of KCl to K₂ SO₄.

Eight decimal twenty-seven (8.27) grams of technical grade MgO was measuredinto the graduated beaker 22 which was filled with water to the 100 ml mark. The small magnet 34 was placed in the slurry 38 and the beaker 22 was put on the electro-magnetic heater and stirrer 36. The beaker 22 was fitted with the rubber bung 20 in which there were three holes: one for tube 4 to the glass sparger 2, one for the thermometer 24 and the third toallow gas to escape along tube 26.

Air containing different amounts of SO₂ (0.1-12%) was successively passed through the MgO slurry 38 until it became a clear solution. The solution was analyzed for MgSO₄, MgSO₃, Mg(HSO₃)₂ and H₂ SO₃ content using titrimetric (caustic, iodine) and gravimetric methods for analysis.

The results given in the following Table I were for experiments carried outon a 100 mls MgO slurry containing 8.27 MgO and an Air-SO₂ flow rate of 1.0 liter/minute. The slurry temperature at the beginning of experiments was 20° C. Entering Air-SO₂ temperature was 20° C. throughout the experiment.

                  TABLE I                                                          ______________________________________                                         EXPERI-                                                                        MENTS    CHARACTERISTICS AND COMPOSITION                                       (SO.sub.2 Concen-                                                                       OF CLEAR LIQUOR OBTAINED                                              tration in                                                                              IN THE EXPERIMENTS                                                    Air-SO.sub.2 Gas                                                                        Temp          H.sub.2 SO.sub.3                                                                     Mg(HSO.sub.3).sub.2                                                                    MgSO.sub.3                                                                           MgSO.sub.4                          Flow)    °C.                                                                             pH    g/l   g/l     g/l   g/l                                 ______________________________________                                         Experiment                                                                     (1) 0.5% (V)                                                                            20      4.0   4.34  39.65   6.67  215.9                               Experiment                                                                     (2) 1.0% (V)                                                                            25      4.0   4.66  69.78   14.40 179.10                              Experiment                                                                     (3) 3.0% (V)                                                                            38      4.5   4.82  166.70  6.87  137.92                              Experiment                                                                     (4) 5.0% (V)                                                                            50      4.0   5.63  115.08  7.38  162.95                              ______________________________________                                    

Referring now to FIG. 2, there is shown a filter flask 42 having a rubber bung 44 through which a valved burette 6 extends together with a pipeline 48, from a source of pressurized air (not shown), and a gas outlet tube 50. The flask 42 contains a magnetic stirrer 51 and is on a heater 52 which also actuates the stirrer 51.

The tube 50 contains a check valve 53 and extends through a rubber bung 54 to a sparger 56 in a beaker 58. A gas outlet tube 60 from the beaker 58 extends through the bung 54 and through a bung 62 into a flask 64. A tube 66 connected to a vacuum source (not shown) also extends through the bung 62. A thermometer 63 extends through the bung 44.

In operation, a slurry 68 from the apparatus shown in FIG. 1 and containingMgSO₄, Mg(HSO₃)₂, H₂ SO₃ and water soluble MgSO₃ is placed in the flask 42, the burette 46 is filled with a solution 70 of H₂ SO₄ and hydrogen peroxide 72 is placed in the beaker 58. The heater 52 is energized and a vacuum is applied through tube66 to the flask 64.

Measured quantities of H₂ SO₄ are released from the burette 70 into the slurry 68 which is heated to a temperature of at least 40°C. to produce MgSO₄ +SO₂ +H₂ O from the slurry 68. SO₂ produced in the flask 42 passes along the tube 50 into the hydrogen peroxide 72 in the beaker 58 where it is converted into H₂ SO₄. Any SO₂ vapour escaping from the beaker 58 along the tube 60 is passed through the filter flask 64 and to the vacuum.

In further tests to verify the present invention, 25 mls of the clear solution, obtained from the experiments using the apparatus shown in FIG. 1, was transferred into the 250 ml filter flask 42 and the flask 42 was fitted with the rubber bung 44. There were three openings in the rubber bung 44: one for the delivery tip of the burette 46, which contained sulphuric acid, the other for the glass tube 48 for aeration and the thirdto hold the thermometer 63.

The filter flask 42 containing the clear solution and the small magnet stirrer 51 was placed on the electrical heater 52 and heated thereby to 60° C. while the stirrer 51 was actuated. The filter flask 42 was attached, by the plastic tube 50, to the glass sparger 56 which was immersed into the closed, tall beaker 58 containing 30 mls hydrogen peroxide. The outlet from the tall beaker 58 was connected, through tube 60 by the other filter flask 64 to the vacuum system 66.

10" (254 mm) mercury vacuum was applied to the system and at least 5% in excess of the stoichiometric requirement of H₂ SO₄ was added to the liquor to decompose MgSO₃ and Mg(HSO₃) to MgSO₄, SO₂ and H₂ O. At the end of the decomposition, when bubble formation in the flask 42 ceased, a small amount of air was passed throughthe liquor to remove SO₂ trapped in the liquor.

The decomposed liquor was tested for excess H₂ SO₄ concentration and then neutralized with MgO. Hydrogen peroxide, containing sulphuric acid, was titrated with 1.0N NaOH and the amount of SO₂ liberated during decomposition was calculated from the test result. The results of the tests are given in the following Table 2, wherein 25 mls of the clear liquors obtained in experiments 1 to 5 above were decomposed with concentrated sulphuric acid and the excess acid neutralized with MgO.

                                      TABLE 2                                      __________________________________________________________________________     EXPERIMENTS                                                                    (SO.sub.2 Concentra-                                                                    SULPHURIC ACID ADDED TO DECOMPOSE CLEAR                               tion in Air-SO.sub.2                                                                    LIQUOR, SO.sub.2 GENERATED DURlNG DECOMPOSITION                       Gas Flow Used in                                                                        AND MgSO.sub.4 CONTENT OF FINAL LIQUOR                                Preparing the                                                                           H.sub.2 SO.sub.4                                                                     SO.sub.2                                                                              MgO   MgSO.sub.4                                                                            Temp                                        Clear Liquor)                                                                           Added (g)                                                                            Liberated (g)                                                                         Added (g)                                                                            g/l pH °C.                                  __________________________________________________________________________     Experiment (1)                                                                 0.5% (V) 0.67  0.86   0.1   249.49                                                                             6.8                                                                               60                                          Experiment (2)                                                                 1.0% (V) 1.28  1.51   0.1   241.82                                                                             7.0                                                                               60                                          Experiment (3)                                                                 3.0% (V) 2.40  3.66   0.1   249.00                                                                             7.1                                                                               60                                          Experiment (4)                                                                 5.0% (V) 1.66  2.27   0.1   246.06                                                                             6.9                                                                               60                                          __________________________________________________________________________

In FIG. 3, there is shown a flow diagram for the removal of SO₂ from, for example, flue or smelter gas on an industrial scale.

In FIG. 3, the flue or smelter gas, with fly ash removed therefrom, is passed sequentially through scrubbers 74 and 76 where the gas is scrubbed with an aqueous slurry of MgO.

The slurry leaving scrubber 74 is collected in a thickener 78 where the solids are separated leaving a clear liquor. The clear liquor, which contains mainly MgSO₄ and different amounts of Mg(HSO₃)₂, MgSO₃ and H₂ SO₃, is passed to a decomposer tank 80 where it is steam heated and treated with concentrated sulphuric acid to decompose Mg(HSO₃)₂, MgSO₃ and H₂ SO₃ to MgSO₄ +SO₂ and water.

The decomposed liquor, which is mainly MgSO₄, is passed to an aerator tank 82 where it is aerated with compressed air to remove SO₂.

S0₂ from the thickener 78, the decomposer 80 and the aerator 82 is passed to a sulphuric acid plant (not shown) for conversion to sulphuric acid.

Magnesium sulphate is pumped from the aerator tank 82 by pump 84 to a neutralizer tank 86 where MgO is added to neutralize any excess H₂ SO₄ present in the solution. MgSO₄ is pumped by pump 87 to a conversion plant (not shown) for further treatment.

Underflow from the thickener 78, which is in the form of a slurry containing unreacted MgO and insoluble MgSO₃, is pumped by pump 88 toa make-up tank 90 where it is diluted with water, preferably that used to initially condition the flue or smelter gas, and fresh MgO is added.

The MgO slurry thus formed is pumped by pump 92 from the make-up tank 90 tothe scrubber 76.

Underflow from scrubber 76 is pumped by pump 92 to scrubber 74.

In FIG. 4, there is shown a flow diagram of an industrial application of the present invention for the removal of SO₂ from a gaseous stream such as, for example, flue gas or non-ferrous smelter gases.

In FIG. 4, the flue or smelter gas is spray scrubbed with water in a conditioner 174 to cool the gas to ambient temperature and remove fly ash therefrom.

The cooled, scrubbed flue gas is then passed through an absorber 176 where it is scrubbed with a slurry of magnesium oxide for the removal of SO₂. Cleaned flue or smelter gas is released to atmosphere from the absorber 176 while the clear solution, containing mainly MgSO₄ and varying amounts of Mg(HSO₃)₂, H₂ SO₃ and soluble MgSO₃, thus obtained is passed to the decomposer 178 wherein it is steam heated to about 40° C. to decompose the H₂ SO₃ to SO₂ and H₂ O. The SO₂ is passed to a sulphuric acid plant 180, where it is converted to sulphuric acid, or it is liquefied at 181 for use in, for example, paper pulp bleaching.

Sulphuric acid is then added in the decomposer 178 in an amount representing at least 5% in excess of the stoichiometric requirement to decompose Mg (HSO₃)₂ and MgSO₃ according to the equations:

    MgSO.sub.3 +H.sub.2 SO.sub.4 →MgSO.sub.4 +SO.sub.2 +H.sub.2 O

    Mg(HSO.sub.3).sub.2 +H.sub.2 SO.sub.4 →MgSO.sub.4 +2SO.sub.2 +2H.sub.2 O

The resulting MgSO₄ solution is then passed to a neutralizer 182 whereexcess H₂ SO₄ present is neutralized with MgO to convert this free acid to MgSO₄.

The neutralized MgSO₄ solution is then passed to a crystallizer 184 where potassium chloride is added to convert the magnesium sulphate to magnesium chloride according to the equation:

    2MgSO.sub.4 +2KCl+6H.sub.2 O→K.sub.2 SO.sub.4.MgSO.sub.4.6H.sub.2 O+MgCl.sub.2

The MgCl₂ thus produced is then removed in centrifuge 186 and passed with water to a neutralizer 188 while the remainder is passed to another crystallizer 190 where water and further potassium chloride is added to produce the reaction according to the following equation:

    K.sub.2 SO.sub.4.MgSO.sub.4.6H.sub.2 O+2KCl→2K.sub.2 SO.sub.4 +MgCL.sub.2 +6H.sub.2 O

The slurry of crystals thus produced is then passed to a centrifuge 192 where the magnesium chloride solution is separated from the potassium sulphate crystals. The magnesium chloride solution is passed to the neutralizer 188 while the potassium sulphate crystals are dried in a drier194 and passed for storage as a saleable product.

As shown, sulphuric acid from the sulphuric acid plant 180 is passed for use in the decomposer 178.

The magnesium chloride, at the neutralizer 188, is treated with calcined dolomite to produce magnesium hydroxide and calcium chloride according to the equation:

    MgCl.sub.2 +CaO.MgO+2H.sub.2 O→2Mg(OH).sub.2 +CaCl.sub.2

The products are passed to a centrifuge 196 where the Mg(OH)₂ solids are separated and portions fed to the absorber 176, the neutralizer 182, and the remainder fed to a calciner 198 for conversion to MgO as a saleable product.

The calcium chloride solution is fed from the centrifuge 196 to an evaporator 198 where it is concentrated to a saturated solution and passedto a crystallizer 200 where the temperature is reduced to about 10° C. to crystallize the calcium chloride.

The solution containing the crystals is passed to a centrifuge 202 where the crystals are separated from the waste liquor and passed to a dryer 204for producing dried calcium chloride as a saleable product.

To summarize, in the method according to the present invention, SO₂ isremoved from a gaseous stream, such as flue gas and non-ferrous smelter gases, the SO₂ is absorbed in an aqueous slurry of magnesium oxide (MgO) 4.0 to 4.5 to produce a clear liquor at a pH in the range of about 3to 5, which contains mainly MgSO₄ and varying amounts of Mg(HSO₃)₂, H₂ SO₃ and soluble MgSO₃ depending on the concentration of SO₂ present in the flue or smelter gas and on the temperature of the scrubbing slurry. The clear liquor is brought to about 40° C. to decompose H₂ SO₃ to SO₂ and H₂ O, sulphuric acid is added in an amount representing at least 5% in excessof the stoichiometric requirement to decompose Mg(HSO₃)₂ and MgSO₃ according to the equations:

    MgSO.sub.3 +H.sub.2 SO.sub.4→MgSO.sub.4 +SO.sub.2 +H.sub.2 O

    Mg(HSO.sub.3).sub.2 +H.sub.2 SO.sub.4 →MgSO.sub.4 +2SO.sub.2 +2H.sub.2 O

If excess H₂ SO₄ is present in the MgSO₄ solution obtained above, the solution should be neutralized with MgO to convert the free acid to MgSO₄.

The sulphur dioxide produced may be converted to sulphuric acid, half of which may be returned to decomposition circuit and the other half may be marketed as a saleable product.

The magnesium sulphate may be converted to magnesium chloride according to the equations:

    2MgSO.sub.4 +2KCl+6H.sub.2 O>K.sub.2 SO.sub.4.MgSO.sub.4.6H.sub.2 O+MgC1.sub.2

    K.sub.2 SO.sub.4.MgSO.sub.4.6H.sub.2 O+2KCl→2K.sub.2 SO.sub.4 +MgCl.sub.2 +6H.sub.2 O

Potassium sulphate crystallizes out of solution and upon drying is ready tobe marketed, for example as fertilizer.

The magnesium chloride may be reacted with calcined dolomite resulting in magnesium hydroxide and calcium chloride according to the equation:

    MgCl.sub.2 +CaO.MgO+2H.sub.2 O→2Mg(OH).sub.2 +CaCl.sub.2

After filtration of the slurry, half of the magnesium hydroxide may be returned to the absorption circuit, while the other half may be calcined to produce saleable MgO. The filtrate may be evaporated and calcium chloride is crystallized out of solution. After drying, the CaCl₂ crystals may be marketed as a saleable products. 

I claim:
 1. A method of removing sulphur dioxide from a gaseous stream, comprising:(a) contacting the gaseous stream with an aqueous slurry of magnesium oxide containing about 3-10%/w of magnesium oxide, so that SO₂ of the gaseous stream is absorbed by the aqueous slurry by converting MgO to an aqueous solution at a pH in the range of about 4.0 to 4.5 containing MgSO₄ and different smaller amounts of Mg(HSO₃)₂, H₂ SO₃ and water soluble MgSO₃. (b) thoroughly mixing sulphuric acid with the aqueous solution at a H₂ SO₃ decomposition temperature of at least 40° C. to produce SO₂ and H₂ O therefrom, the sulphuric acid being present in an amount representing at least 5% in excess of the total stoichiometric requirement of the aqueous solution to decompose Mg(HSO₃)₂ and MgSO₃ according to the reactions:

    MgSO.sub.3 +H.sub.2 SO.sub.4 →MgSO.sub.4 +SO.sub.2 +H.sub.2 O

    Mg(HSO.sub.3).sub.2 +H.sub.2 SO.sub.4 →MgSO.sub.4 +2SO.sub.2 +2H.sub.2 O,

(c) converting the sulphur dioxide produced in step 9b) to sulphuric acid by catalytic reaction with H₂ O and O₂ according to the reaction:

    SO.sub.2 +1/2O.sub.2 +H.sub.2 O→H.sub.2 SO.sub.4

wherein at least a portion of the H₂ SO₄ thus produced is recycled to the reaction of step (b).
 2. The method according to claim 1, wherein magnesium sulphate produced in step (b) is converted to magnesium chloride by thoroughly mixing therewith potassium chloride and H₂ O according to the following two stage process:(i) adding potassium chloride to produce the following reaction:

    2MgSO.sub.4 +2KCl+6H.sub.2 O→K.sub.2 SO.sub.4.MgSO.sub.4.6H.sub.2 O+MgCl.sub.2

whereby the potassium sulphate, in the form of a double salt, is separated from the, MgCl₂ and then, (ii) adding additional potassium chloride to the separated double salt suspended in water to produce the following reaction:

    2KCL+K.sub.2 SO.sub.4.MgSO.sub.4.6H.sub.2 O→2K.sub.2 SO.sub.4 +MgCl.sub.2 +6H.sub.2 O,

then separating the K₂ SO₄, which has crystallized from the solution, from the remainder.
 3. A method according to claim 2, wherein calcined dolomite is added to the remainder to produce the following reaction:

    MgCl.sub.2 +CaO.MgO+2H.sub.2 O→2Mg(OH).sub.2 +CaCl.sub.2 (aqueous solution),

and the magnesium hydroxide separated from the calcium chloride aqueous solution.
 4. A method according to claim 3, wherein a portion of the magnesium hydroxide is formed into an aqueous slurry and is recycled for use as the aqueous slurry of magnesium hydroxide in step (a), and the remaining magnesium hydroxide is calcined to produce MgO therefrom.
 5. A method according to claim 3, wherein a portion of the H₂ O is removed from the calcium chloride aqueous solution to crystallize calcium chloride in the remaining H₂ O, and then the crystallized calcium chloride is separated from the remaining H₂ O and dried.
 6. A method according to claim 1, wherein the step (b) is carried out under a reduced pressure.
 7. A method according to claim 1, wherein the aqueous magnesium oxide slurry contains about 8.27 %/w of magnesium oxide.
 8. A method according to claim 1, wherein the gaseous stream is at ambient temperature.
 9. A method according to claim 10, wherein fly ash has been removed from the gaseous stream. 